Crystalline molecular sieve layers and processes for their manufacture

ABSTRACT

A process is described for the manufacture of crystalline molecular sieve layers with good para-xylene over meta-xylene selectivity&#39;s good para-xylene permeances and selectivities. The process requires impregnation of the support prior to hydrothermal synthesis using the seeded method and may be undertaken with pre-impregnation masking. The crystalline molecular sieve layer has a selectivity (alphax) for para-xylene over meta-xylene of 2 or greater and a permeance (Qx) for para-xylene of 3.27x10&lt;-8 &gt;mole(px)/m&lt;2&gt;.s.Pa(px) or greater measured at a temperature of &gt;=250° C. and an aromatic hydrocarbon partial pressure of &gt;=10x10&lt;3 &gt;Pa.

This invention relates to crystalline molecular sieve layers, toprocesses for their manufacture, and to their use.

Molecular sieves find many uses in physical, physicochemical, andchemical processes; most notably as selective sorbents, effectingseparation of components in mixtures, and as catalysts. In theseapplications the crystallographically-defined pore structure within themolecular sieve material is normally required to be open; it is then aprerequisite that any structure-directing agent, or template, that hasbeen employed in the manufacture of the molecular sieve be removed,usually by calcination. Numerous materials are known to act as molecularsieves, among which zeolites form a well-known class.

In International Application WO 94/25151 is described a supportedinorganic layer comprising optionally contiguous particles of acrystalline molecular sieve, the mean particle size being within therange of from 20 nm to 1 μm. The support is advantageously porous. Whenthe pores of the support are covered to the extent that they areeffectively closed, and the support is continuous, a molecular sievemembrane results; such membranes have the advantage that they mayperform catalysis and separation simultaneously if desired. A number ofprocesses are described in WO 94/25151 for the manufacture of theinorganic layers disclosed therein. WO94/25151 describes the use of abarrier layer which prevents the water in the aqueous coating suspensionused from preferentially entering the pores of the support to an extentsuch that the silica and zeolite particles form a thick gel layer on thesupport. The barrier layer may be temporary or permanent; temporarybarrier layers are fluids such as water or glycol. The membranes of WO94/25151 exhibited selectivities of para-xylene over ortho-xylene of20.76 to 60.10 and para-xylene permeances of 1.09×10⁻⁸mole(px)/m².s.Pa(px) (10 kg(px)/m².day.bar(px)) when measured at lowtemperature and pressure.

In International Application WO 96/01683 a structure is described whichcomprises a support, a seed layer, and an upper layer, the seed layercomprising a crystalline molecular sieve having a crystal size of atmost 1 μm, and the upper layer comprising a crystalline molecular sieveof crystals having at least one dimension greater than the dimensions ofthe crystals of the seed layer. There are a number of processesdescribed in WO 96/01683 for the manufacture of these layers.

In International Application WO 97/25129 a structure is described whichcomprises a crystalline molecular sieve layer on a substrate and anadditional layer of refractory material to occlude voids in themolecular sieve layer. The structures described in the examples havepara-xylene over meta-xylene selectivities of between 2 to 8.

In International Application WO 96/01686 a structure is described whichcomprises a substrate, a zeolite or zeolite-like layer, a selectivityenhancing coating in contact with the zeolite layer and optionally apermeable intermediate layer in contact with the substrate. Examples ofthese structures are given which have para-xylene over meta-xyleneselectivities of between 1 to 10.

Xomeritikas and Tsapatsis in Chemical Materials, 1999, 11, 875-878,describe orientated MFI-type zeolite membranes which have beenmanufactured using secondary growth a process which requires twosuccessive hydrothermal growths and produces membranes of 25 to 40 μmthickness. These membranes exhibited para-xylene over ortho-xyleneselectivities of 18 when measured at a total aromatic hydrocarbonpartial pressure of 27.5 Pa [=15 Pa pX+12.5 Pa oX] and 100° C. and 3.8at a total aromatic hydrocarbon partial pressure of 550 Pa [=300 PapX+250 Pa oX] and 100° C., and permeances for para-xylene of 2.0 to5.2×10⁻⁸ mole/m².s.Pa [18 to 48 kg_(px)/m².day.bar_(px)], when tested attemperatures up to 200° C. and at low hydrocarbon partial pressures. Theselectivity decreased with increasing partial pressure of para-xyleneand it was observed by the authors that the membranes would not besuitable for separation of xylene isomers at elevated temperatures dueto the 20 fold reduction in flux ratio at 200° C. compared to thatobserved at 100° C.

Many commercial petrochemical processes operate at elevated temperatureand pressure. Whilst the molecular sieve layers of the prior art mayexhibit good selectivity and permeance results when tested at lowtemperatures, pressures and/or hydrocarbon partial pressures, this isnot repeated when tested at high temperatures and high hydrocarbonpartial pressures. Thus, there is a need for molecular sieve layers withimproved properties for catalytic and/or membrane applications,especially improved properties at elevated temperatures e.g. >250° C.and/or elevated hydrocarbon feed partial pressures >10×10³ Pa.

The present invention is concerned with crystalline molecular sievelayers which have improved properties compared to crystalline molecularsieve layers in the art, especially for membrane applications. It hassurprisingly been found that the control of a number of synthesisparameters for the manufacture of crystalline molecular sieve layers inconjunction with impregnation of the support onto which the crystallinemolecular sieve layer is to be deposited during its synthesis, resultsin crystalline molecular sieve layers with properties, which hithertohave not been achieved.

The present invention in a first aspect provides a process for themanufacture of a crystalline molecular sieve layer, which processcomprises:

a) providing a porous support having deposited thereon seeds ofmolecular sieve crystals of average particle size of 200 nm or less,

b) impregnating the support with an impregnating material before orafter deposition of the seeds of molecular sieve,

c) contacting the impregnated support having seeds deposited thereonwith a molecular sieve synthesis mixture,

d) subjecting, the impregnated support having seeds deposited thereon,to hydrothermal treatment whilst in contact with the molecular sievesynthesis mixture to form a crystalline molecular sieve layer on thesupport, and

e) removing the impregnating material from the support.

As examples of porous supports, there may be mentioned porous glass,sintered porous metals, e.g., steel or nickel, inorganic oxides, e.g.,alpha-alumina, titania, cordierite, zeolite as herein defined, orzirconia and mixtures of any of these materials. In this context poroussupports include supports which have pores which are occluded; suchsupports, whilst having pores which are not suitable for membraneseparation applications, may be used for catalytic applications orseparation processes which are not membrane separation processes such asfor example adsorption or absorption.

The pore size and porosity of the support should be compatible with theprocess employed for depositing the molecular sieve seeds. The poroussupport may be any material compatible with the coating and synthesistechniques utilised in the process of the present invention. For exampleporous alpha-alumina with a surface pore size within the range of 0.08to 1 μm, most preferably from 0.08 to 0.16 μm, and advantageously with anarrow pore size. Ideally the support should have a relatively highdegree of porosity so that the support exerts an insignificant effect onflux through the finished product. Preferably the porosity of thesupport is 30% by volume or greater; ideally and preferably greater than33%, and preferably within the range 33 and 40% by volume. The supportmay be multilayered; for example, to improve the mass transfercharacteristics of the support; in this context the support may be anasymmetric support. In such a support the surface region which is incontact with the molecular sieve seeds may have small diameter pores,while the bulk of the support, toward the surface remote from themolecular sieve seeds, may have larger diameter pores. An example ofsuch a multilayered asymmetric support is an alpha-alumina disk havingpores of about 1 μm average diameter coated with a layer ofalpha-alumina with average pore size of about 0.1 μm. A further exampleof a multilayered support is a large pore metal based support which hasan inorganic layer (either metal or non-metal) deposited thereon ofsmaller pore size compared to the metal support. It is to be understoodthat when the support is a molecular sieve as herein defined and atleast at its surface it has the requisite properties to function as amolecular sieve seed, in relation to particle size and crystallinity,then the support surface itself may act as the molecular sieve seed anddeposited molecular sieve seeds may be dispensed with. Zeolite supportsmay however also be used in conjunction with a deposited molecular sieveseeds. Suitable supports include the composite membranes and layersmanufactured according to U.S. Pat. Nos. 4,981,590 and 5,089,299.

It is preferred that the support is such that it is substantially inertunder hydrothermal reaction conditions. It is preferred thatsubstantially no chemical component of the support participates in themolecular sieve synthesis and, as a result, becomes incorporated withinthe structure of the crystalline molecular sieve layer. This isparticularly advantageous when the crystalline molecular sieve layer isto function as a catalyst material or to act as the support for acatalyst material. In these circumstances, incorporation of unwantedchemical species into the structure of the crystalline molecular sievelayer may be detrimental to these functions. In addition, if thecrystalline molecular sieve layer is to be used as a membrane,incorporation of unwanted chemical species from the support into thelayer may adversely affect the permeation properties of the layer.

The support may be, and preferably is, cleaned prior to deposition ofthe molecular sieve seeds. Suitable cleaning techniques includeultrasonic treatment in water, pentane, acetone or methanol. This may befollowed by a period of drying from a few minutes to 24 hours underambient conditions or under temperatures up to 1000° C., preferably 500to 700° C. The cleaning regime may comprise a combination of cleaningsteps. Such a combination may be a series of washing steps withdifferent solvents and/or drying steps. Each solvent washing step may beutilised in combination with ultrasound.

The molecular sieve seeds may be deposited, and preferably aredeposited, as a discrete layer, or part of a discrete layer, whichcomprises molecular sieve seed crystals of average particle size 200 nmor less. Advantageously, the average crystal size of the molecular sieveseeds in the seed layer is 150 nm or less ideally within the range 5 to120 nm and most preferably within the range 25 to 100.

The seed layer may consist substantially of molecular sieve materialonly, or it may be a composite layer of the molecular sieve seedmaterial and intercalating material which may be organic or inorganic.The particles of the seed layer may be contiguous or non-contiguous;preferably they are contiguous. The intercalating material may be thesame material as the support. The preferred molecular sieve seedcrystals are colloidal in nature and capable of forming a stablecolloidal suspension.

Colloidal molecular sieve seed crystals may be prepared by processes,which are well known in the art. Suitable processes are those describedin International Applications; WO93/08125, WO97/03019 WO97/03020WO97/03021 and WO94/05597, the disclosures of which, in so far as theyrefer to the manufacture of colloidal molecular sieve seeds, areincorporated by reference.

The molecular sieve seed may be applied to the support by techniquesknown in the art such as for example sol-gel coating techniques,spin-coating, wash-coating, spray-coating, brushing, slip-casting ordip-coating; these processes preferably being undertaken with asuspension of the colloidal molecular sieve crystals.

The colloidal molecular sieve seed crystals are preferably applied tothe support by spin-coating; the viscosity of the mixture, the solidsconcentration and the spin rate inter alia controlling the coatingthickness. The mixture may firstly be contacted with the stationarysupport, then after a short contact time the support is spun at thedesired rate. Alternatively, the mixture is contacted with a supportwhich is already spinning at the desired rate.

When present as a discrete layer, the thickness of the molecular sieveseed layer is advantageously 3 μm or less, more advantageously at most 2μm, preferably 1 μm or less and most preferably 0.5 μm or less.Advantageously, the seed layer is of sufficient thickness to coverirregularities of comparable scale in the surface of the support.Advantageously, the seed layer is at most the thickness of thesubsequently deposited crystalline molecular sieve layer.

In one embodiment the seed layer may be deposited and used as amonolayer. Such a monolayer and its method of deposition is described inWO97/33684, the disclosure of which in so far as it relates to themanufacture of a molecular sieve seed monolayer is incorporated byreference. It is preferred that the molecular sieve seed layer is onethat has substantially a monolayer thickness. It is preferred that thismonolayer is deposited via the charge reversal method utilising acationic polymer as described in WO97/33684.

In one aspect of the process of the present invention the support may beimpregnated and placed into the molecular sieve synthesis mixturewithout further treatment of the molecular sieve seed layer after itsdeposition. Even when submerged in the synthesis mixture, the particlesin the seed layer remain adhered to the support and facilitate growth ofthe zeolite layer. However, under some circumstances, e.g. duringstirring or agitation of the synthesis mixture, the adhesion between themolecular sieve seed layer and the support may be insufficient and stepsmay be taken to stabilise the seed layer.

Therefore, in another aspect of the invention, the molecular sieve seedlayer is stabilised before impregnation or before being placed into thesynthesis mixture. This stabilisation can be achieved in one aspect byheat-treating the seed layer, e.g. at temperatures between 30 and 1000°C., ideally greater than 50° C. and more preferably between 200° C. and1000° C. and most preferably greater than 300° C. and between 400° C.and 600° C., for several hours preferably at least two hours and mostpreferably 2 to 10 hours.

The impregnating material may be any material which substantiallyremains at its selected location within the support during subsequentprocess steps used for deposition of the crystalline molecular sievelayer e.g. hydrothermal synthesis conditions, and deposition of themolecular sieve seed layer if this occurs after impregnation, and whichis substantially stable under such process condition, at least for thetime scale of the process.

The impregnation material selected must remain substantially within thesupport, and must remain substantially stable, under the depositionconditions so as not to interfere with the deposition process and toensure that a crystalline molecular sieve layer of the desired qualityand properties is obtained in the process.

Ideally the impregnation material should have a viscosity which enableseasy impregnation into the support. The properties of the impregnationmaterial ideally are such that it may be impregnated into the supportunder capillary action, applied pressure or a vacuum. Furthermore, theimpregnation material should be compatible with the physical propertiesof the support surfaces to ensure that it can wet the surfaces of thesupport and intimately contact with it.

Water and glycol are not suitable as impregnation material because theydo not remain at any location in the support, selected for theimpregnating material, under hydrothermal synthesis conditions.

The impregnation material should also be capable of being easily andsubstantially completely removed from the support after formation of thecrystalline molecular sieve layer. Ideally at least the bulk of theimpregnating material is capable of being removed under an appliedpressure, by washing of the support with a suitable solvent, viacalcination, via melting or any combination of these methods. It ispreferred that the impregnation material is capable of being removedunder calcination conditions which are normally used in the manufactureof molecular sieve materials such as those used in zeolite synthesis. Itis important that the impregnation material can easily be removed inorder to ensure that as little residual impregnation material aspossible, and preferably no residual impregnation material, remainswhich could impair the performance of the crystalline molecular sievelayer.

The preferred impregnation materials include natural or syntheticorganic resins e.g hydrocarbon resins. In the context of the presentinvention hydrocarbon means an organic material which has as its maincomponents hydrogen and carbon but does not preclude the presence of oneor more heteroatomic species e.g. oxygen or nitrogen or chlorine. Onepreferred class of impregnating material are the hydrocarbon resinswhich are free of heteroatoms. If a heteroatom is present it ispreferred that it is oxygen or chlorine. Examples of suitable resins areacrylic resins, PVC resins and the hydrocarbon waxes.

Examples of suitable acrylic resins are the L R White Resinsmanufactured and supplied by the London Resin Co. These are hydrophilicacrylic resins of low viscosity (typically 8 mPa.s) which arecommercially available in three grades of hardness; LR1280 hard grade,LR1281 medium grade and LR1282 soft grade. These resins may be thermallyor cold cured, with or without the use of an accelerator such as LR1283.

Suitable hydrocarbon resins include for example the hydrocarbon waxessuch as Exxon ESCOMER™ H101 and H231. H101 has a molecular weight withinthe range 1600 to 2300 and a viscosity at 121° C. of approximately 25.5mPa.s, at 140° C. of approximately 17 mPa.s and at 190° C. ofapproximately 9 mPa.s. H231 has an approximate molecular weight of 6590and a viscosity at 121° C. of approximately 600 mPa.s.

An example of a suitable impregnating material incorporating PVC is aPVC plastisol. Such plastisols are well known in the art and typicallycomprise PVC in combination with plasticizer, stabiliser and viscositydepressor.

Further examples of suitable impregnating materials areethylene-butylene resins of approximate molecular weight 300 to 10000 orpolyisobutylene resins of approximate molecular weight 500 to 5000.

The molecular sieve seed material may be deposited prior to or afterimpregnation of the support; preferably in one embodiment it isdeposited prior to impregnation of the support. In this instance afterimpregnation of the support there may be quantities of impregnatingmaterial located on the surface of the molecular sieve seed layer, whichhas already been deposited on the support. If this layer of impregnatingmaterial is relatively thin or discontinuous then surprisingly it maynot have an adverse effect on the seeding properties of the molecularsieve seed layer and need not be removed or if some removal is desiredneed not be completely removed. This is especially the case where theimpregnating material is mildly unstable under the conditions used forsubsequent deposition of the crystalline molecular sieve layer e.g.hydrothermal synthesis conditions, and is slowly dissolved in thesynthesis mixture. Such a material, in accordance with the requirementsof the process of the present invention, has acceptable stability.Examples of materials which have this property include, the hydrocarbonwaxes, acrylic resins and ethylenelbutene resins described above. Ifnecessary excess impregnation material may be removed from the surfaceof the molecular sieve seed layer by any suitable means. One suitablemeans, in the case where a co-solvent is used for impregnation, is touse the same solvent to clean the surface of the seed layer. When noco-solvent is used then any suitable solvent for the resin may be usedto clean the seed layer surface. The thickness of this surface depositedlayer of impregnation material should be less than 1 μm and preferablyit should be less than 0.5 μm, and most preferably less than 0.1 μm

The most preferred resins are the hydrocarbon wax resins which mayeasily impregnate the support and which are removed from the supportunder calcination temperatures that are normally used in zeolitesynthesis, with or without prior melting of the bulk material.

Materials which have been found to be unsuitable as impregnatingmaterials include some low molecular weight hydrocarbons e.g hexadecane,silicone oils and polyimide resins. This is believed to be mainly due totheir propensity for relatively rapid removal from the support under theconditions used for deposition of the crystalline molecular sieve layer.

Any suitable impregnation material may be used alone or in combinationwith other impregnation materials and/or other materials which may berequired to assist in their impregnation. For example PVC resins mayadvantageously be impregnated into the support as a solution in THF; theTHF being evaporated prior to deposition of molecular sieve seed layerand/or crystalline molecular sieve layer. Other suitable solvents may beused in conjunction with the resins. The resins may be applied in themolten form under ambient pressure conditions or under an appliedpressure; for example hydrocarbon waxes are advantageously applied inthe molten form.

The impregnation stage may be and preferably is repeated one or moretimes to ensure that the pores of the support, which are at or proximateto the surface for deposition of the molecular sieve seed layer orcrystalline molecular sieve layer, are substantially filled withimpregnating material. Alternatively impregnation may be undertaken forextended periods of time to achieve the same result as repeatedimpregnation stages. In the case of hydrocarbon wax as impregnatingmaterial the impregnation time is typically in the order of 2 minutes ormore at 150° C. under vacuum, ideally 2 to 5 minutes; for the samematerial an extended impregnation time is greater than 5 minutes andideally in the order of 20 minutes or more under the similar conditions.Wax impregnation may usefully be, and preferably is, undertaken for onehour or more at 150° C. under an applied vacuum.

In one embodiment the support is impregnated through surfaces of thesupport other than the surface onto which the crystalline molecularsieve layer is to be deposited. For example a support in the form of adisk may be impregnated through one side only; the other side being thesurface onto which the molecular sieve seed layer and crystallinemolecular sieve layer are to be deposited. In one embodiment, theimpregnation may be partial in order to fill the pores of the surfacesother than the surface onto which the crystalline molecular sieve layeris to be deposited. This partial filling of the pores of the support isacceptable if it results in improved performance of the crystallinemolecular sieve layer compared to that manufactured withoutimpregnation. Partial impregnation is particularly suitable when amolecular sieve seed layer is used and the crystalline molecular sievelayer is deposited via hydrothermal synthesis utilising a zeolitesynthesis solution which comprises colloidal silica. Surprisingly thecombination of a seed layer and colloidal silica in the synthesissolution, allows the use of partial impregnation. Impregnation may becontinued until substantially all the pores of the support areimpregnated including pores proximate to the surface of the molecularsieve seed. In the case of wax impregnation this may be observedvisually by an optical change in the support and the degree ofimpregnation can be confirmed by cross-section SEM. In a furtherembodiment the support may be impregnated through the molecular sieveseed layer.

After impregnation the nature of the organic resin may be such that itis advantageous to cure the resin in-situ prior to use of theimpregnated support in the manufacture of a crystalline molecular sievelayer. This curing ensures that the resin remains in the impregnatedlocation during subsequent manufacture of the crystalline molecularsieve layer. Advantageously and preferably the impregnating material hasa melting point at or above the temperature used in the process formanufacture of the crystalline molecular sieve layer. It is notessential that the impregnating material is or remains solid within thesupport during manufacture of the crystalline molecular sieve layer. Itmay become liquid or molten during this manufacture; this is acceptableif in this physical state the impregnating material meets therequirements described in detail above i.e. remains stable and in thedesired location within the support.

In a further aspect the process of the present invention may utilise afurther process step which is undertaken prior to impregnation of theporous support. When used in conjunction with impregnation thisadditional process step provides further control in the process andfurther improvements in performance and ease of manufacture. Thisfurther process step may be referred to as pre-impregnation masking andinvolves deposition of a removable coating onto the support surfacewhich in due course will receive the crystalline molecular sieve layer.The pre-impregnation masking step enables a more accurate and effectimpregnation stage to be undertaken. The pre-impregnation masking isapplied to the appropriate surface of the porous support such that itdoes not impregnate the support or only impregnates, to a limitedextent, the surface region of the support. After deposition of thepre-impregnation masking the support is then impregnated as describedabove, ideally so that the impregnating material comes into contact withor close proximity to the pre-impregnation masking. Once impregnation iscompleted the pre-impregnation masking may be removed and the remainingprocess steps undertaken in order to manufacture the crystallinemolecular sieve layer.

The pre-impregnation masking may be applied before or after depositionof a molecular sieve seed layer on the support. When applied to asupport which already has a molecular sieve seed layer deposited on itssurface the pre-impregnation masking offers the additional benefit ofprotecting the molecular sieve seed layer surface from contaminationwith the impregnating material. When the pre-impregnation masking isapplied to a support which does not have a seed layer deposited on itssurface the seed layer is advantageously applied after removal of thepre-impregnation masking onto a high quality impregnated support. Ofparticular benefit is the use of such a high quality impregnated supportwith the monolayer seeding method described in WO97/33684. When thisseeding method is used in conjunction with pre-impregnation masking goodquality crystalline molecular sieve layers may be produced.

An important factor in pre-impregnation masking is to ensure that thematerial used for the masking is able to intimately contact the surfaceof the support and is compatible with the impregnating material andmethod of impregnation. If contact properties are inadequateimpregnating material may fill spaces which arise between thepre-impregnation masking and the support; the resultant region ofimpregnating material on the support surface prevents subsequentdeposition and growth of the crystalline molecular sieve layer and canthus lead to a poor quality layer.

The steps required for pre-impregnation masking include; cleaning of thesupport surface, coating the support surface with an appropriate maskingmaterial, impregnation of the masked support and removal of the maskingmaterial after impregnation.

The methods used to clean the support surface may be the same as thoseindicated above for preparation of the support for impregnation. Apreferred method is to rinse the support in acetone and filtered ethanol(0.1 μm filter, Anotop™ Whatman) followed by drying.

The material used for the pre-impregnation masking may be any materialwhich can be easily applied to the surface of the support and which maybe readily removed after impregnation without significant disturbance tothe impregnation material. The pre-impregnation masking material must becompatible with the surface of the porous support so as to effectivelywet and coat substantially the whole of the desired region for masking.The choice of pre-impregnation masking material will also depend on thenature of the support e.g. its surface properties such as polarity.Examples of suitable pre-impregnation masking materials include organicpolymers. Of particular interest for the masking of inorganic andasymmetric supports such as ceramics, in particular alpha-alumina, arepolar polymeric materials such as the acrylic polymers and resins. Apreferred masking material is polymethylmethacrylate (PMMA). An exampleof a suitable PMMA polymer is CM205 of MW 100,000 g/mole with apolydispersity of 1.8. An example of a polymer which is less suitablefor use as a masking material with asymmetric alpha-alumina supports ispolystyrene; it is believed that this is due to its relatively lowpolarity. Preferred organic polymers therefore have a polarity which isgreater than that of polystyrene. The masking material may be applied ina number of ways. One method is to melt the organic polymer and to applythis to the surface of the support. A further and preferred method is toapply the organic polymer from solution in a suitable solvent for thepolymer. In this context a true solution may not be formed and thesolvent simply reduces the viscosity of the masking material for ease ofapplication. A particularly useful solvent for PMMA is acetone.Preferably, the PMMA as masking material is applied as a solution of 1part PMMA in 3.75 parts acetone. The solution of masking material isapplied to the support and the deposited material is carefully dried toremove the solvent if used. Too rapid a drying process may lead toineffective masking. In the case of PMMA applied via acetone the solventis removed by drying at a rate of 1° C./h to 150° C. Impregnation of themasked support may be undertaken as described above.

After impregnation, the pre-impregnation masking material is removed. Asuitable method for removal is washing with a suitable solvent. In thecase of PMMA and other polar masking materials, a suitable solvent isacetone or the solvent that was used in the application of the mask.After solvent removal of the masking material the impregnated supportsurface that was in contact with the masking material may be furthertreated and preferably is further treated with an ammonia solution,ideally a 0.1M ammonia solution. After this treatment the impregnatedsupport may be utilised for the deposition of a seeding layer,preferably using the monolayer technique, and deposition of acrystalline molecular sieve layer.

The composition of the synthesis solution is selected to provide thedesired molecular sieve or molecular sieve type. When the crystallinemolecular sieve layer comprises silicon in its framework then the H₂O toSiO₂ ratio must be within the range of 7 to 100. Preferred siliconsources include tetraethylorthosilicate (TEOS) and colloidal silica whenthe support is partially impregnated. Preferably, the H₂O to SiO₂ molarratio in the synthesis mixture is within the range of 7 to 70, morepreferably 7 to 60. For certain molecular sieves such asaluminophosphates (ALPO's) a source of silica is not required.

The composition of the synthesis mixture varies according to theprocess; the mixture always contains sources of the various componentsof the desired molecular sieve and usually contains a structuredirecting agent. A preferred colloidal silica source is anammonia-stabilised colloidal silica, e.g., that available from du Pontunder the trade mark Ludox AS-40.

The source of silicon may also be the source of potassium, in the formof potassium silicate. Such a silicate is conveniently in the form of anaqueous solution such, for example, as that sold by Aremco Products,Inc. under the trade mark CERAMA-BIND, which is available as a solutionof pH 11.3, specific gravity 1.26, and viscosity 40 mPas. Other sourcesof silicon include, for example, silicic acid.

As other sources of potassium, when present, there may be mentionedpotassium hydroxide. Whether or not the synthesis mixture contains apotassium source, it may also contain sodium hydroxide to give thedesired alkalinity.

The structure directing agent, when present, may be any of thosecommonly used in zeolite synthesis. For the manufacture of an MFI layer,a tetrapropylammonium hydroxide or halide is advantageously used.

For the manufacture of an MFI type zeolite, especially ZSM-5 orsilicalite-I, the synthesis mixture is advantageously of a molarcomposition, calculated in terms of oxides, within the ranges:

M₂O:SiO₂ 0 to 0.7 to :1 preferably 0 to 0.350:1, SiO₂:Al₂O₃ 12 toinfinity :1 (TPA)₂O:SiO₂ 0 to 0.2:1 preferably 0 to 0.075:1 H₂O:SiO₂ 7to 100:1 preferably 9 to 70:1

wherein TPA represents tetrapropylammonium and M is an alkali metal,preferably sodium or potassium, although it may also be Li , Cs orammonia. Other template agents may be used in these ratios. In theembodiment where pre-impregnation masking is not used its is mostpreferred that the M₂O:SiO₂ molar ratio is within the range 0.016 to0.350:1, and preferably that the that the H₂O:SiO₂ molar ratio is withinthe range 7 to 60, more preferably 9 to 30:1, and most preferably 9 to20:1.

In this specification ratios with infinity as the value indicate thatone of the ratio materials is not present in the mixture.

The hydrothermal synthesis is preferably undertaken at a temperature ofbetween 60 and 180° C. and for a period within the range 1 to 200 hours.In a preferred aspect the process of the present invention utilises ahydrothermal synthesis temperature of 140° C. or less, preferably withinthe range from 60 to 100° C., and most preferably within the range 60 to90° C. When pre-impregnation masking is used the preferred temperaturerange is 60 to 100° C.

In a preferred aspect the process of the present invention utilises asynthesis time of 4 to 100 hours, in particular 4 to 80 hours and mostpreferably 4 to 36 hours. The time of reaction will vary depending onthe temperature used during the hydrothermal synthesis and may beadjusted accordingly with shorter synthesis times generally beingapplicable when higher synthesis temperatures are used.

In the most preferred aspect of the process the hydrothermal synthesistemperature is approximately 90° C., the hydrothermal synthesis time isapproximately 36 hours, and the H₂O:SiO₂ molar ratio in the synthesismixture is within the range 9 to 20.

The hydrothermal treatment advantageously is undertaken in an autoclaveunder autogenous pressure. However, with synthesis temperatures below100° C. it is possible to perform the synthesis under ambient pressureconditions.

After deposition of the crystalline molecular sieve layer theimpregnating material is substantially completely removed by any of themethods or combination of methods indicated above. The removal methodchosen will depend to some extent on the exact nature of theimpregnating material. The essential requirement is that the removalmethod is capable of removing substantially all of the impregnatedmaterial. One suitable method is to utilise the final calcination stepin the molecular sieve synthesis process to remove the impregnatingmaterial.

After crystallisation, the structure comprising the support anddeposited crystalline molecular sieve layer with or without impregnatingmaterial may be washed, dried, and the crystalline molecular sievecalcined. The calcination conditions preferably comprise slow heatingand cooling to ensure that the structure, and in particular thecrystalline molecular sieve layer, remains intact with the minimumamount of cracking and/or delamination. Preferably, the structure iscalcined at a temperature of 350 to 600° C., preferably 450 to 550° C.It is preferred that the structure is raised to the desired calcinationtemperature at a rate of 0.1 to 6° C. per minute most preferably 0.2 to3° C. per minute.

In relation to the processes described herein contacting is to beunderstood to include immersion or partial immersion of the support inthe relevant zeolite synthesis mixture.

The crystalline molecular sieve layer may be any known molecular sievematerial; for example it may be a silicate, an aluminosilicate, analuminophosphate (ALPO's), a silicoaluminophosphate, ametalloaluminophosphate, or a metalloaluminophosphosilicate.

The preferred molecular sieve will depend on the chosen application,e.g. separation, catalytic applications, and combined reaction andseparation, and on the size of the molecules being treated. There aremany known ways to tailor the properties of the molecular sieves, forexample, structure type, chemical composition, ion-exchange, andactivation procedures.

Representative examples are molecular sieves/zeolites which may be usedin the molecular sieve layer include the structure types AFI, AEL, BEA,CHA, EUO, FAU, FER, KFI, LTA, LTL, MAZ, MOR, MEL, MTW, OFF, TON and,especially and preferably MFI.

The structure types of the molecular sieve seed and crystallinemolecular sieve layers may be the same or different. Further, if thestructure types are the same, the compositions may be the same ordifferent. It is preferred that the molecular sieve seeds and thecrystalline molecular sieve layer are both of the MFI structure type.

Some of the above materials while not being true zeolites are frequentlyreferred to in the literature as such, and this term will be usedbroadly in this specification.

It is preferred that the hydrothermal synthesis stage of the process isundertaken under such conditions as to prevent the settling, on theforming crystalline molecular sieve layer, of particles produced withinthe synthesis mixture e.g. molecular sieve crystals which havehomogeneously nucleated in the synthesis solution. Contacting of thesupport coated with molecular sieve seeds is advantageously carried outby immersion or partial immersion and with the support in an orientationand location in the synthesis mixture such that the influence ofsettling of crystals formed in the reaction mixture itself, rather thanon the coated surface, is minimised. If support surface is threedimensional, e.g., a honeycomb, other means may be used to inhibitsettling, for example, agitation, stirring or pumping.

The process of the present invention provides crystalline molecularsieve layers with good separation properties especially at hightemperatures ≧250° C. and preferably ≧360° C. and/or hydrocarbon feedpartial pressures in the feed of ≧50×10³ Pa, preferably ≧100×10³ Pa,most preferably at 500×10³ Pa. Crystalline molecular sieve layers,especially when in the form of a membrane, have been characterised bymeans of a number of analytical techniques. One such technique is thedye permeation test as described in WO96/01683. Whilst this test is agood indication as to whether or not unacceptable defects are present ina crystalline molecular sieve layer, it is a coarse test and filter, anddoes not provide any absolute measurable difference which isquantifiable between different crystalline molecular sieve layers whichpass the test; it is a pass or fail test. Crystalline molecular sievelayers have been further characterised using x-ray diffraction,transmission electron microscopy (TEM) and scanning electron microscopy(SEM). Such techniques have been used to characterise crystallinemolecular sieve membranes in for example WO96/01683.

The crystalline molecular sieve layers of the present invention whencharacterised using the dye permeation test or SEM are indistinguishablefrom those crystalline molecular sieve layers described in WO96/01683.However, it has been found that the crystalline molecular sieve layersof the present invention exhibit different membrane properties,especially at high temperature and/or hydrocarbon partial pressure,compared to the prior art. It is possible to characterise thecrystalline molecular sieve layers of the present invention using asimple membrane test which measures the transport characteristics, suchas the selectivity and mass transport properties of the crystallinemolecular sieve layer. This test enables the crystalline molecular sievelayers of the present invention to be distinguished from the prior artlayers.

The test method is based on the evaluation of the selectivity andpermeance of the crystalline molecular sieve layer arranged in the formof a membrane, using for example a mixture comprising para-xylene andmeta-xylene or para-xylene and ortho-xylene; para-xylene and meta-xyleneare particularly suitable to evaluate MFI molecular sieve membranes. Thecrystalline molecular sieve layer as a membrane is first analysed forits capacity to preferentially transport para-xylene from a mixturecomprising para-xylene and meta-xylene on the feed side of the membraneto the permeate side of the membrane. The permeance of each isomer ismeasured simultaneously and the ratio of para-xylene to meta-xylenepermeance provides a selectivity for para-xylene over meta-xylene. Thisparameter is dimensionless. The details of the test and calculations ofselectivity and permeance are provided in the examples below.

It has been found that the crystalline molecular sieve layers of thepresent invention have good para-xylene over meta-xylene selectivity andpermeance, especially at high temperatures and aromatic hydrocarbonpartial pressures.

Accordingly the present invention also provides a crystalline molecularsieve layer having a selectivity (α_(x)) for para-xylene overmeta-xylene of 2 or greater and a permeance (Q_(x)) for para-xylene of3.27×10⁻⁸ mole(px)/m².s.Pa(px) (30 kg(px)/m².day.bar(px)) or greatermeasured at a temperature of ≧250° C. and an aromatic hydrocarbonpartial pressure of ≧10×10³ Pa.

The selectivity and permeance are calculated and determined as describedbelow. The crystalline molecular sieve layers of the present inventionare defined in terms of their selectivity and permeance properties forpara-xylene separations. However, the present invention is not limitedto crystalline molecular sieve layers only when used for para-xyleneseparations; the layers may be used for other separations and/orapplications such as catalysts and sensors e.g. gas sensors. For α_(x)and Q_(x) the subscipt x denotes the total aromatic hydrocarbon partialpressure in kpa on the feed side of the layer; thus x has a minimumvalue of 10 or greater, preferably 50 or greater, more preferably 100 orgreater and most preferably 500 or greater. Ideally, x is within therange 10 to 1000, more preferably 100 to 1000, and most preferably 500to 1000. Preferably the aromatic hydrocarbon partial pressure is≧100×10³ Pa, more preferably ≧500×10³ Pa. Preferably, the temperature is≧360° C. and most preferably ≧4000, and ideally within the range 250° C.to 600° C., most preferably within the range 360° C. to 600° C. It ispreferred for all layers of the present invention that the performancelevels are attained in the presence of hydrogen.

It is preferred that the para-xylene over meta-xylene selectivity(α_(x)) of the membrane layer is 2.5 or greater, more preferably 3 orgreater, more preferably 5 or greater and most preferably 8 or greater.Ideally, it is within the range 2 to 30000, preferably 8 to 3000, andmost preferably 8 to 100. The para-xylene permeance (Q_(x)) ispreferably 5.45×10⁻⁸ mole(px)/m².s.Pa(px) (50 kg(px)/m².day.bar(px)) orgreater, and more preferably 7.63×10⁻⁸ mole(px)/m².s.Pa(px) (70kg(px)/m².day.bar(px)) or greater. Ideally, it is within the range3.27×10⁻⁸ to 5.4×10⁻⁶ mole(px)/m².s.Pa(px) (30 to 5000kg(px)/m².day.bar(px)), more preferably within the range 7.63×10⁻⁸ to3.3×10⁻⁶ mole(px)/m².s.Pa(px) (70 to 3000 kg(px)/m².day.bar(px)).

It will be appreciated that the structure comprising a crystallinemolecular sieve layer and a support may be of any shape, and may be, forexample, planar, cylindrical, especially cylindrical with a circularcross-section, or may be a honeycomb structure. For clarity, however,the following description will refer to the structure as if it wereplanar, and references will be made to the plane of a layer.

The products of the invention may additionally be characterised by X-RayDiffraction (XRD) among other techniques. For this purpose aconventional X-Ray diffraction technique may be used, where thesupported layered structure in the shape of a disk is mounted in amodified powder sample holder and a conventional θ/2θ scan is performed.The intensities of the zeolite reflections thus measured are compared tothe intensities of reflections of a randomly oriented powder of azeolite of the same structure and composition. If one or more sets ofreflections related to one or more specific orientations of the crystalare significantly stronger than the remaining reflections as compared tothe diffractogram of a randomly oriented powder, this indicates that theorientation distribution in the sample deviates from random. This isreferred to as a crystallographic preferred orientation or CPO. It ispreferred that the crystalline molecular sieve layers of the presentinvention are MFI structure type molecular sieves and exhibit a strongcombined 011/101 reflection which is indicative of 011/101 CPO asmeasured by X-Ray Diffraction (XRD).

The thickness of the crystalline molecular sieve layer is advantageouslyless than 3 μm, more advantageously less than 2 μm, and preferably 1 μmor less most preferably 0.5 μm or less. Advantageously, the thickness ofthe layer, and the crystallite size of the crystalline molecular sieve,are such that the layer thickness is approximately the size of thelongest edges of the crystals, giving essentially a monolayer. In such amonolayer the crystals are orientated such that the crystallinemolecular sieve layer exhibits a columnar appearance when viewed incross-section by SEM. In such a structure the majority of theinter-crystal grain boundaries are oriented substantially perpendicularto the plane which approximates to the interface between the support andcrystalline molecular sieve layer. The crystalline molecular sieve layercontains substantially no crystals which are orientated such that theplane of their grain to grain interfaces are parallel to thesupport/crystalline molecular sieve layer interface; without wishing tobe bound to any theory, the inventors believe that such interfacess mayreduce the performance of the membrane.

It is preferred that the combined thickness of the molecular sieve seedlayer and the crystalline molecular sieve layer is 3 μm or less,preferably 2 μm or less, and most preferably 1 μm or less.

Advantageously, in the hydrothermally deposited crystalline molecularsieve layer, the crystals are contiguous, i.e. substantially everycrystal is in contact with one of its neighbours, although notnecessarily in contact with one of its neighbours throughout its entirelength.

Although it is desired that the crystalline molecular sieve layers ofthe present invention are crack free as determined by the dye test. Itis acceptable to have cracks which may be reparated. It is alsoacceptable for the surface of the crystalline molecular sieve layer toexhibit a significant degree of surface cracking. It is surprising thatalthough the crystalline molecular sieve layers of the present inventionmay exhibit an extensive surface cracked topography, they still exhibitgood selectivity, and permeance even without the use of reparationtechniques.

The crystalline molecular sieve layers of the present invention may betreated to further improve or stabilise their properties. In one aspect,whilst intact layer regions are of good quality, there may be regions ofthe layer which are cracked or where there may be pinholes present. Ifthese cracks and pinholes are of such quantity and dimensions that theyhave a disproportionate effect on membrane performance then it is usefulto reparate the layer. Suitable reparation techniques are described infor example WO96/01682, WO96/01686 and WO97/25129, the disclosures ofwhich are incorporated by reference. The preferred method of reparationis that described in WO96/01686. If the crystalline molecular sievelayer of the present invention has no pinholes or cracks whichdisproportionately effect the layer performance it may still beadvantageous to treat the crystalline molecular sieve layer to maintainits performance. In this context a suitable treatment is the selectivityenhancing layers described in WO96/01686. Such selectivity enhancinglayers may at the same time also reparate defective crystallinemolecular sieve layers. Such selectivity enhancing layers providemechanical stability to the crystalline molecular sieve layers duringuse.

An alternative reparation method involves the use of a hydrolysedcrystallisation solution. In this method a hydrolysed synthesis mixture,identical or similar to that used to deposit the crystalline molecularsieve layer, is applied to the surface of the crystalline molecularsieve layer on the support. Any suitable application method may be used;one such method is spin-coating at for example 8000 rpm. Afterdeposition of the hydrolysed synthesis mixture the surface of thecrystalline molecular sieve is further treated with an ammonia solutione.g. 0.1 M ammonia to clean the surface. The treated and ammonia cleanedcrystalline molecular sieve layer is then exposed to moisture atelevated temperature, ideally in a closed autoclave at 100° C. for 24hours. After exposure to moisture the crystalline molecular sieve iscalcined. A suitable calcination regime is heating to 400° C. in air for6 hours using a heat-up and cool-down rate of 2° C. per minute. Thisreparation method is particularly suitable for reparation of crystallinemolecular sieve layers which have been manufactured using thepre-impregnation masking method.

When a membrane layer is reparated the end result is typically amodification of the selectivity and permeance properties of the layer.Typically there is a reduction in the permeance and an increase in theselectivity. It has surprisingly been found that when crystallinemolecular sieve layers manufactured by the process of the presentinvention are reparated they possess high selectivity for para-xyleneover meta-xylene and in addition retain acceptably good para-xylenepermeance properties.

Accordingly the present invention in a further embodiment provides areparated membrane comprising crystalline molecular sieve and having aselectivity (α_(x)) for para-xylene over meta-xylene of 10 or greaterand a permeance (Q_(x)) for para-xylene of 4.36×10⁻⁹mole(px)/m².s.Pa(px) (4 kg(px)/m².day.bar(px)) or greater measured at atemperature of ≧250° C. and an aromatic hydrocarbon partial pressure of≧10×10³ Pa.

Preferably, the reparated membrane layer has a para-xylene overmeta-xylene selectivity (α_(x)) of 12, most preferably 17 and ideally 60or 100 or greater. Preferably, the selectivity is within the range 10 to30000, more preferably 10 to 3000, and most preferably 10 to 200. Thesubscript x has the same values and ranges as indicated above fornon-reparated membranes.

Preferably the reparated membrane layer has a para-xylene permeance(Q_(x)) of 5.12×10⁻⁹ mole(px)/m².s.Pa(px) or greater (4.7kg(px)/m².day.bar(px)), more preferably 7.08×10⁻⁹ mole(px)/m².s.Pa(px)or greater (6.5 kg(px)/m².day.bar(px)) more preferably 8.1×10⁻⁹ orgreater, and most preferably 1.09×10⁻⁸ mole(px)/m².s.Pa(px) or greater(10 kg(px)/m².day.bar(px)). Preferably the permeance is within the rangeof 6.54×10⁻⁹ to 5.4×10⁻⁶ mole(px)/m².s.Pa(px) (6 to 5000kg(px)/m².day.bar(px)), and most preferably within the range 7.0×10⁻⁹ to3.3×10⁻⁶ mole(px)/m².s.Pa(px).

In a further aspect the process of the present invention producescrystalline molecular sieve layers which may be characterised by afurther aspect of their separations performance. It has been found thatthe selectivity of para-xylene over meta-xylene is not constant withtime during use but surprisingly increases in a specific way, which isbeneficial. This effect may be used to attain, maintain or improve thedesired permeance and selectivity performance. Without being bound byany theory it is believed that these crystalline molecular sieve layershave a morphology and structure which lends itself to this effect.However, these morphology and structure differences cannot bedistinguished from prior art crystalline molecular sieve layers due tothe limitations of available analytical techniques. It is believed thatthese differences allow the crystalline molecular sieve layers of thepresent invention to preserve their selectivity performance during useand to allow this selectivity performance to be easily improved. Whenthe crystalline molecular sieve layers manufactured by the process ofthe present invention are exposed to a mixed hydrocarbon stream e.g. anaromatics stream it is believed that some component or components of thestream reduce the detrimental effects of non-selective pathways throughthe crystalline molecular sieve layer whilst having little or no effecton the selective pathways. This is in effect a form of reparation whichoccurs during use of the crystalline molecular sieve layer and which maybe controlled during use. As would be expected the permeance ofindividual components of the hydrocarbon mixture reduces with time ofexposure. However, it is surprising that for some components thereduction is significantly more than for others. This selectivereduction is believed to account for the improved selectivity. Thereduction in permeance of key components is not detrimental to theoverall performance of the crystalline molecular sieve layer if at thesame time there is a consequential improvement in selectivity. Theselectivity improvement is particularly noticeable for the separation ofpara-xylene from an aromatics stream. This effect may be observed byusing the xylenes separation test described above to provide a plot ofselectivity for the desired component e.g. para-xylene against time.This plot will show that the selectivity for para-xylene overmeta-xylene increases with time. If this data plot is modified toexpress the first differential (Δα_(Px/Mx)/Δt), (where α=the selectivityfor para-xylene over meta-xylene at a given time t), averaged over thefirst two hours of testing, then the value of this differential at t=2is >0 i.e. it is increasing.

Thus the present invention in a further aspect provides a membranecomprising a crystalline molecular sieve layer, which membrane has a(Δα_(Px/Mx)/Δt) at t=2 hours of greater than 0.

It has further been observed that as time of exposure to the hydrocarbonfeed is extended then the selectivity either remains constant or mayslowly and gradually decrease. It has been found that this effect may beprevented or reversed by controlling the hydrocarbon partial pressure inthe feed to the crystalline molecular sieve layer during use. If thehydrocarbon partial pressure in the feed to the crystalline molecularsieve layer is increased this surprisingly has been found to improve theselectivity for selected components of the feed.

Accordingly in a further aspect the invention provides a process forenhancing the selectivity of a crystalline molecular sieve layer for theseparation of at least one component from a hydrocarbon stream whichprocess comprises:

a) exposing a crystalline molecular sieve layer to a hydrocarbon streamcomprising at least two components under pressure for a period of time,such that at least one component is separated from the stream, and

b) at some point in time during the exposure increasing the partialpressure of at least one component of the hydrocarbon stream.

The increase in hydrocarbon partial pressure, which increase is on thefeed side of the crystalline molecular sieve layer, may be a gradualincrease which occurs throughout the separation cycle or it may be agradual increase for a proportion of the cycle or it may be a steppedincrease in pressure or a combination of these. The increase in pressuremay be applied one or more time during the process if desired. Thisprocess may be utilised to enhance the performance of crystallinemolecular sieve layers so that they meet the desirable performancetargets of a selectivity for para-xylene over meta-xylene of 2 orgreater and a permeance for para-xylene of 3.27×10⁻⁸mole(px)/m².s.Pa(px) (30 kg(px)/m².day.bar(px)) or greater at atemperature of ≧250° C. and an aromatic hydrocarbon partial pressure of≧10×10³ Pa.

The preferred crystalline molecular sieve layers for use in this processare those prepared by the process described above for the manufacture ofcrystalline molecular sieve layers.

The preferred hydrocarbon stream is an aromatics stream and mostpreferably is an aromatics stream which comprises a mixture of xyleneswith other aromatic components.

Processes suitable for operation in accordance with this aspect of theinvention are described in for example WO93/08125, WO97/03019 WO97/03020WO97/03021 and WO94/05597 and as described below.

The invention also provides a structure in which the support, especiallya porous support, has crystalline molecular sieve layers according tothe invention on each side, the layers on the two sides being the sameor different; it is also within the scope of the invention to provide alayer not in accordance with the invention on one side of the support,or to incorporate other materials in the support if it is porous.

A catalytic function may be imparted to the crystalline molecular sievelayers of the invention either by bonding of a catalyst to the supportor the free surface of the crystalline molecular sieve layer, or itslocation within a tube or honeycomb formed of the structure, by itsincorporation in the support, e.g., by forming the support from amixture of support-forming and catalytic site-forming materials or inthe seed layer or crystalline molecular sieve layer itself. If thesupport is porous a catalyst may be incorporated into the pores, thecatalyst optionally being a zeolite. For certain applications, itsuffices for the structure of the invention to be in close proximity to,or in contact with, a catalyst, e.g. in particulate form on a face ofthe structure.

The crystalline molecular sieve layer may be configured as a membrane, aterm used herein to describe a barrier having separation properties, forseparation of fluid (gaseous, liquid, or mixed) mixtures, for example,separation of a feed for a reaction from a feedstock mixture, or incatalytic applications, which may if desired combine catalysedconversion of a reactant or reactants and separation of reactionproducts. The crystalline molecular sieve layer may be removed from thesupport on which it is formed for use as a membrane or catalyst. Thismay be achieved by methods known in the art including for exampledissolution of the support. It is preferred that the crystallinemolecular sieve layers of the present invention are supported on aporous support in use ideally the support used for their manufacture.

Conversions which may be effected include isomerizations, e.g., ofalkanes and alkenes, conversion of methanol or naphtha to alkenes,hydrogenation, dehydrogenation, e.g., of alkanes, for example propane topropylene, oxidation, catalytic reforming or cracking and thermalcracking.

The present invention accordingly also provides a process for theseparation of a fluid mixture which comprises contacting the mixturewith one face of a structure according to the invention in the form of amembrane under conditions such that at least one component of themixture has a different steady state permeability through the structurefrom that of another component and recovering a component of mixture ofcomponents from the other face of the structure.

The present invention accordingly also provides a process for theseparation of a fluid mixture which comprises contacting the mixturewith a structure according to the invention in one embodiment in theform of a membrane under conditions such that at least one component ofthe mixture is removed from the mixture by adsorption. Optionally theadsorbed component is recovered and used in a chemical reaction or maybe reacted as an adsorbed species on the structure according to theinvention.

The invention further provides such processes for catalysing a chemicalreaction in which the structure is in close proximity or in contact witha catalyst.

The invention further provides a process for catalysing a chemicalreaction which comprises contacting a feedstock with a structureaccording to the invention which is in active catalytic form undercatalytic conversion conditions and recovering a composition comprisingat least one conversion product.

The invention further provides a process for catalysing a chemicalreaction which comprises contacting a feedstock with one face of astructure according to the invention, that is in the form of a membraneand in active catalytic form, under catalytic conversion conditions, andrecovering from an opposite face of the structure at least oneconversion product, advantageously in a concentration differing from itsequilibrium concentration in the reaction mixture.

The invention further provides a process for catalysing a chemicalreaction which comprises contacting a feedstock with one face of astructure according to the invention that is in the form of a membraneunder conditions such that, at least one component of said feedstock isremoved from the feedstock through the structure to contact a catalyston the opposite side of the structure under catalytic conversionconditions.

The invention further provides a process for catalysing a chemicalreaction which comprises contacting one reactant of a bimolecularreaction with one face of a structure according to the invention, thatis in the form of a membrane and in active catalytic form, undercatalytic conversion conditions, and controlling the addition of asecond reactant by diffusion from the opposite face of the structure inorder to more precisely control reaction conditions. Examples include:controlling ethylene, propylene or hydrogen addition to benzene in theformation of ethylbenzene, cumene or cyclohexane respectively.

The crystalline molecular sieve layers of the present invention haveparticular utility in the separation of para-xylene from mixturescomprising paraxylene and at least one other isomer of xylene.Accordingly the present invention also provides a process for theseparation of para-xylene from a mixture comprising para-xylene and atleast one other isomer of xylene, which process comprises exposing themixture to a crystalline molecular sieve layer according to the presentinvention at a temperature of ≧250° C. and an aromatic feed partialpressure of ≧10×10³ Pa. In this embodiment the aromatic feed is a feedwhich comprises isomers of xylene optionally with other aromatichydrocarbons e.g. ethylbenzene. Alternative pressures and temperaturesas indicated above in respect of the crystalline molecular sieve layersmay also be used in this process.

The following Examples, in which parts are by weight unless indicatedotherwise, illustrate the invention:

EXAMPLES 1 to 4 Preparation of Alumina Porous Support

Porous alumina supports were cleaned as follows:

1. Ultrasonicate in water for 10 minutes.

2. Heat treat in air overnight at 700° C.

3. Ultrasonicate in pentane for 10 mins.

4. Remove and dry in air for 10 minutes.

5. Ultrasonicate in acetone for 10 minutes.

6. Remove and dry in air for 10 minutes.

7. Ultrasonicate in methanol for 10 minutes.

8. Remove and dry for approximately 2 hours at 110° C. and cool to roomtemperature.

Preparation of Colloidal Seeds

Silicalite colloidal seeds of ˜50 nm particle size were preparedaccording to the general method as described in WO93/08125.

Deposition of Seed Layer

The colloidal seeds were deposited on the cleansed alumina supports byspin-coating, a colloidal suspension of 0.5% by weight of ˜50 nm sizedMFI crystals with a pure silica composition and as prepared above.

A porous alpha-alumina disk of diameter 25 mm, thickness 3 mm, pore size80 nm, and ˜33% porosity by volume, was machined and polished on oneface. The disk was then placed in the specimen chuck of a CONVAC ModelMTS-4 spinner and brought up to a spinning speed of 4000 rpm. Once thisspinning speed had been reached 2 ml of the seed solution was applied tothe centre of the disk and spinning was continued to a total of 30seconds. The coated disk was placed in an oven and heated up to atemperature of 425° C. or 450° C. at a heat-up rate of 0.3° C./min andheld at the terminal temperature for 6 hours. After 6 hours the coateddisk was cooled at a rate of 0.5° C./min until the disk reached roomtemperature.

Impregnation

A petri dish was partially filled with H101 hydrocarbon wax, which hadbeen melted at 150° C. in a vacuum oven. The porous support, with seedlayer deposited thereon, was placed on a holder in the wax filled petridish such that only the surface of the support which was free ofdeposited seeds was submerged in the wax. This ensured that the seedlayer did not come into contact with the wax. The vacuum pump wasswitched on and after 2 minutes it was switched off at a vacuum of <50mbar. The oven was brought to atmospheric pressure and the impregnatedwax was allowed to crystallise within the pores of the support. Thisimpregnated support was now ready for deposition of a crystallinemolecular sieve layer.

Preparation of Hydrothermal Synthesis Solutions

A solution was prepared of 0.92 g NaOH (98.4% purity) in 138.14 g ofwater. Into this solution was dissolved 7.12 g of tetrapropylammoniumbromide (TPABr: Fluka). To this mixture was added 76.66 g of colloidalsilica solution (Ludox AS 40, supplied by Du Pont) and the resultingmixture was stirred with a magnetic stirrer for 2 to 10 minutes. Theresulting molar composition was as follows:

0.22 Na₂O:0.52 TPABr:10 SiO₂:200 H₂O

Hydrothermal Synthesis

The impregnated support with seed coating was mounted in a holder withthe spin-coated face pointing downwards, near the surface of thesynthesis mixture in an autoclave. The autoclave was closed, placed inan oven, and heated during 30 minutes to the crystallisation temperatureand maintained at that temperature for the period specified in thefollowing Table. The oven was then allowed to cool to room temperature.After cooling, the disk was removed and washed in demineralized wateruntil the conductivity of the last washing water was ≦5 μS/cm. The diskwas then dried in an oven at 125° C. After drying the resultingstructure was calcined and tested for para-xylene separationsperformance. The calcination conditions were sufficient to removesubstantially all the impregnating material.

Selectivity Enhancing Coating

One crystalline molecular sieve layer was treated using the proceduredescribed in Example 3 of WO96/01686 to provide a selectivity enhancinglayer on the crystalline molecular sieve layer. The resulting structurewas also tested for para-xylene separation performance.

Para-xylene Separation Test

A simplified diagram of a unit used to test the crystalline molecularsieve membrane layers is shown in FIG. 1. Hydrogen feed (1) andaromatics feed (2), are mixed, preheated and vaporised inside a sandbath (7). A hydrogen sweep (3) is also preheated in the sand bath (7).The hydrogen feed (1) combined with aromatics feed (2) flow into thefeed side (8) of a stainless steel cell (11) containing the crystallinemolecular sieve layer on a porous support (9). The hydrogen sweep (3)flows into the same stainless steel cell but into the sweep side (10).This cell is designed such that selected components from the aromaticsfeed pass through the membrane from the feed side into the sweep side atprocess conditions. A product stream labelled retentate (4), which isthe feed depleted of select components, and permeate (5), which is thesweep enriched with selected components from the feed, separately, butsimultaneously, flow out of the stainless steel cell. The permeate (5)is analysed by an on-line chromatograph (GC) (6), and the composition ofthe permeate is used in conjunction with the permeate flow to calculatethe flow of each individual component through the membrane.

Following is a detailed description of the testing procedure.

1. A molecular sieve membrane on a porous support is mounted into ametal (steel) cell and sealed with a graphoil o-ring. It is preferableto have the surface of the steel cell passivated so that it does notinduce catalytic cracking and coking reactions in the test. Thecatalytic activity of the cell and the membrane assembly can be assessedby measuring the level of cracking products in the permeate. It is alsopreferable to pretreat the graphoil o-ring so that it does not outgasscarbonaceous materials which have the potential of fouling the membraneand reducing observed xylenes flows through the membrane. One procedurefor pretreating graphoil o-rings is by heating up under air at 450° C.for 3 h followed by cooling to room temperature. It should be noted thatthe graphoil o-ring is applied directly to the zeolite layer or anyselectivity-enhancing coating or reparation layer if applied.

2. The cell with the membrane mounted inside is then heated to atemperature of at least 250° C. and ideally between 360 and 400° C. Asuitable heating rate is ˜2° C./min. While the membrane is being heated,hydrogen is flowed across the feed and sweep side of the membrane. Flowrates for tests with a ˜2.5 cm diameter membrane sealed with a graphoilgasket which exposes an area of 2.91 cm² to the feed are:

100 ml/min at 100×10³ Pa absolute on the feed side

100 ml/min at 100×10³ absolute on the sweep side

It should be noted that the feed side is the side of the membranestructure sealed by the graphoil gasket (i.e., the side on which thecrystalline molecular sieve layer is deposited). In this steady statethere is no Δp across the membrane.

For the ˜2.5 cm diameter membrane, a liquid hydrocarbon mixture which,is inter alia composed of para-xylene and meta-xylene isomers isintroduced at a rate of 33 ml/h into the hydrogen flowing on the feedside of the membrane. The line carrying the mixture to the cell passesthrough a hot zone in order to ensure that the feed is vaporised and tobring the mixture to the temperature at which the test is to beconducted. The pressure on the feed side is then increased by at least50×10³ Pa, ideally at least 100×10³ Pa absolute. This provides a Δpacross the membrane of at least 50×10³ Pa and ideally at least 100×10³Pa.

At the testing temperatures, the hydrogen partial pressure on the feedside is approximately equal to the hydrogen partial pressure in theflowing hydrogen sweep stream. With this testing procedure, hydrogentransference through the membrane is minimised and there is said to behydrogen balance.

The composition of the aromatic hydrocarbon mixture used in the exampleswas nominally 70% meta-xylene (mX), 20% para-xylene (pX), 5%ethylbenzene (EB), and 5% trimethyl-benzene (TMB) by weight; variationin this composition is acceptable. In the context of the presentinvention reference to aromatic hydrocarbon partial pressure is to thepartial pressure of a mixture of meta-xylene (mX), para-xylene (pX),ethylbenzene (EB), and trimethyl-benzene (TMB). It is preferable thatthe oxygen level in these mixtures be low to prevent chemical reactionswhich can lead to coking. This can be done by degassing the mixtures, orby formulating the mixtures from oxygen free solvents.

The composition of the hydrocarbons in the permeate stream is measuredwith an FID detector in a gas chromatograph. The integrated area foreach component is used to deduce the flux of each component; theintegrated area can be related to the mass fraction of a component inthe permeate by a calibration procedure in which a known concentrationof mixture components is passed through the gas chromatograph.

The values selected for characterising the membrane may be taken at anumber of pressures and temperatures. Any membrane when tested at atemperature of at least 250° C. and a pressure of at least 10×10³ Pa,which has the required permeance and selectivity is a membrane accordingto the present invention. The performance of the membrane is monitoredwith time. The test reading may be taken at any time after the membraneis at the required temperature and pressure and after introduction ofthe hydrocarbon feed. It may be desirable to delay the reading until themembranes selectivity properties are relatively stable. The performancereadings are taken as the maximum values for Q and α attained during thetest. As indicated above membranes prepared according to the process ofthe present invention may exhibit an improvement in selectivityproperties during the initial stages of use; these improvements can berapid or may take extended periods of time to stabilise. This effect isreferred to as selectivation. This initial increase in performancefollowed by a period of relative stability is illustrated in FIG. 2(b),which shows the selectivity properties beginning to plateau afterapproximately 6 hours. This effect may not be observed with reparatedmembranes. The maximum performance values for these membranes may occurat the start of the test; performance values for these membranes aretypically taken early in the test in the first few hours of testing. Theexact point in time at which the test reading is taken will thereforevary with the temperature and test pressures used. What is important isthat the test result is taken at the maximum performance values and inthe case of membranes which exhibit selectivation, when the maximumplateau is reached. Ideally this test result is taken at least 1 hourafter introduction of the hydrocarbon feed. In these experiments, thetest readings were taken at various times between 1 to 20 hours afterintroduction of the hydrocarbon feed. For the reparated membrane thetest result was taken at 1 hour. Hydrogen flow rates are measured inpermeate and retentate. It is preferred that the performance of themembranes is achieved at hydrogen balance.

The permeance of hydrocarbon component A is calculated as follows:${{Permeance}\quad {of}\quad A} = \frac{{Mass}\quad {Flow}\quad {Rate}\quad {of}\quad A\quad {in}\quad {Permeate}}{\left( {{Partial}\quad {Pressure}\quad {of}\quad A\quad {in}\quad {Feed}\text{-}{Partial}\quad {Pressure}\quad {of}\quad A\quad {in}\quad {Sweep}} \right)*{Area}}$

Area=Membrane area exposed to feed by graphoil gasket

Permeance is expressed in SI units as mole(px)/m².s.Pa(px) wheremole(px) refers to moles of para-xylene, and Pa(px) refers to theparaxylene partial pressure in Pa (an example of a non-SI unit oftenused in industry is kg(px)/m².day.bar(px) which is equal to 1.09×10⁻⁹mole(px)/m².s.Pa(px).

Under certain circumstances, the transfer of hydrocarbons through themembrane from feed to sweep is low enough that the partial pressure ofhydrocarbons in the sweep is negligible (note that hydrocarbons are notadded to the sweep so any hydrocarbons present in the permeate must flowthrough the membrane). In such circumstances, one may opt to neglect thepartial pressure of hydrocarbon A in the sweep and calculate thepermeance of A using the partial pressure of hydrocarbon A in the feedas the total transmembrane pressure driving force. The error in suchapproximation is equal to the ratio of the partial pressure of A in thesweep to the partial pressure of A in the feed. Thus, it follows that ifthe partial pressure of A in the sweep is much lower than the partialpressure of A in the feed, the error is low. Using the flow rates givenhere to test the membranes described in this invention, the partialpressure of each hydrocarbon in the sweep is less than five percent ofthe partial pressure of the same hydrocarbon in the feed. This is theresult of having deliberately set the flow rates to attain low transferof hydrocarbons from feed to sweep during testing. The total transfer ofhydrocarbons from feed to sweep was kept at less than five percent theamount of hydrocarbons in the feed. It is preferable that this amount beless than one percent of the amount of hydrocarbons in the feed. Underthese conditions, the partial pressure of hydrocarbons in the sweep wereneglected in the calculation of permeance, and the permeances reportedhere were calculated using the partial pressure of hydrocarbons in thefeed as the total transmembrane pressure driving force. The results arequoted as Q_(x) (Q_(x)=pXy permeance in mole(px)/m².s.Pa(px) and α_(x)(α_(x)=pXy/mXy selectivity) with x indicating the total aromatichydrocarbon partial pressure in kPa. The test parameters used in theseexperiments are as indicated below:

Q_(100 and) α₁₀₀

For Examples 1 to 3 these values were measured under the followingconditions:

Temperature = 360° C. Feed rate =≧ 100 ml/min Sweep rate =≧ 100 ml/min

The composition of feed and sweep gases, in kPa partial pressure of thegases was as follows:

Feed/ Component of Partial Pressure Sweep Composition kPa Comments Feedpara-xylene 25 Total aromatic hydrocarbon meta-xylene 65 partialpressure = 100 kPa ethyl-benzene 5 tri-methyl- 5 benzene Hydrogen 100 nonet flow of H₂ from feed Sweep Hydrogen 100 to sweep or vice versa

For Example 4 these values were measured under the following conditions:

Temperature = 400° C. Hydrogen Feed rate =≧ 120 ml/min Sweep rate =≧ 200ml/min

The composition of feed and sweep gases, in kPa partial pressure of thegases was as follows:

Feed/ Component of Partial Pressure Sweep Composition kPa Comments Feedpara-xylene 20 Total aromatic hydrocarbon meta-xylene 70 partialpressure = 100 kPa ethyl-benzene 5 tri-methyl- 5 benzene hydrogen 120 nonet flow of H₂ from feed to sweep or vice versa Sweep hydrogen 115

Q_(500 and) α₅₀₀

These values were measured under the following conditions:

Temperature = 360-400° C. Feed rate =≧ 200 ml/min Sweep rate =≧ 200ml/min

The composition of feed and sweep gases, in kPa partial pressure of thegases:

Feed/ Component of Partial Pressure Sweep Composition kPa Comments Feedpara-xylene 125 total aromatic hydrocarbon meta-xylene 325 partialpressure = 500 kPa ethyl-benzene 25 tri-methyl- 25 benzene hydrogen 1200no net flow of H₂ from feed Sweep hydrogen 1200 to sweep or vice versa

It must also be pointed out that because of the low total transfer ofhydrocarbons from feed to sweep, the partial pressure of hydrocarbons inthe feed is constant across the membrane surface. Because of this andthe fact that the partial pressures of hydrocarbons in the sweep arenegligible and uniform across the sweep side of the membrane, thepartial pressure difference of each hydrocarbon across the membrane isconstant across the entire membrane area. Therefore, the permeancesreported here are considered point permeances to distinguish them frompermeances one can observe in large-area membranes where theconcentrations in both feed and sweep sides are allowed to vary acrossthe total membrane area (i.e., the transmembrane pressure differencevaries across the membrane area). Such is the case of a large membranemodule, where, if one applies the equation of permeance as written, thepermeance obtained would be an average permeance in the membrane module.One may refer to this permeance as an integrated or module permeancewhich would be different than the point permeances provided here. Theimportance of differentiating between a point permeance and an averageor module permeance is that a point permeance is the parameter one mustuse in engineering the design of a membrane module. An average or modulepermeance, on the other hand, only applies to that specific membranemodule under the testing conditions used.

The selectivity of a component A over a component B is calculated asfollows:${{Selectivity}\quad \frac{A}{B}} = \frac{{Permeance}\quad {of}\quad A}{{Permenace}\quad {of}\quad B}$

The selectivities and permeance values for the layers according to thepresent invention are provided in Table 1.

Examples 1 and 2 illustrate the effect of the additional step ofreparation in the process of the present invention. Example 2 isreparated and exhibits exceptionally high selectivity for para-xyleneover meta-xylene whilst maintaining an acceptable permeance. Theadditional examples illustrate the good selectivities and permeancesobserved with the layers of the present invention.

Supported crystalline molecular sieve layers prepared as described abovewere monitored for the separation performance with time. FIGS. 2(a) and2(b) illustrate the results. FIG. 2(a) shows that after the initialperiod of stabilisation with time, the permeances of the componentsthrough the layer reduces with exposure to the feed. In FIG. 2(a) astepped increase in the feed partial pressure for hydrogen and for thexylenes components of the feed is identified at 1000 minutes ofexposure. In FIG. 2(b) it can bee seen that this stepped increase inpartial pressure has provided a stepped increase in selectivity forpara-xylene over meta-xylene and trimethylbenzene. In addition FIG. 2(b)illustrates a surprising improvement in the selectivity of the layer topara-xylene during use.

TABLE 1 Molecular Sieve Test Time Seed Layer Layer Combined Q_(x) 10⁻⁸Temp on oil Thickness Thickness Thickness mole_(px)/ Example ° C. hoursμm μm μm m².s.Pa_(px) α_(x) Comments 1 360 6 0.5 0.5 1 Q₁₀₀ = 5.8 α₁₀₀ =3.2  Selectivation 2 360 1 0.5 0.5 1  Q₁₀₀ = 0.81 α₁₀₀ = 103  NoSelectivation 3 360 6 0.5 0.5 1 Q₁₀₀ = 5.8 α₁₀₀ = 2.8  Selectivation 4400 6 0.5 0.5 1 Q₁₀₀ = 9.6 α₁₀₀ = 5.4  Selectivation

EXAMPLE 5 Support Cleaning

An α-alumina disk, 2.5 mm diameter, 3 mm thickness, with bulk 3 μm poresize, and several intermediate layers, with a top layer of 100 nm poresize (available from Inocermic GmbH, Hermsdorf, Germany) was cleaned byrinsing in acetone and filtered ethanol (0.1 μm filter, Anotop™,Whatman) and dried.

Pre-impregnation Masking

A solution of 1 part by weight PMMA (polymethylmetacrylate distributedby Polykemi AB, Ystad, Sweden as CM 205, MW 100,000 g/mole,polydispersity 1.8) in 3.75 parts by weight acetone, was passed througha 0.1 μm pore filter (Anotop™, Whatman), and was brought onto the topsurface of the support by using a syringe, filter and needle. Thedeposited solution was then carefully dried by first drying at roomtemperature overnight and then heating with a rate of 1° C./h to 150° C.

Support Impregnation

Hydrocarbon wax (H101 wax) was impregnated from the back of the maskedsupport support for 1 hour at 150° C. under an applied vacuum.

Removal of Pre-impregnation Masking

After impregnation the PMMA coating was removed by washing in acetone,ethanol and the support was then washed with a filtered (0.1 μm filter,Anotop™, Whatman) 0.1 M ammonia solution.

Deposition of Molecular Sieve Seeds

The impregnated support was soaked in a 0.4% aqueous cationic polymersolution (Redifloc 4150, Eka Nobel AB, Sweden pH adjusted to 8.0 by theaddition of ammonia) which was prepared from distilled and filtered (0.1μm filter, Anotop™, Whatman) water and filtered through a 0.8 μm filter(Acrodisc™, Pall) immediately before use. The soaking time was 10minutes. The support was subsequently rinsed 4 times with a filtered(0.1 μm filter, Anotop™, Whatman) 0.1 M ammonia solution. The supportwas immersed for 10 minutes in a sol containing 1% ˜60 nm silicalite-1crystals. The pH of the sol was 10.0. The support was subsequentlyrinsed 4 times with a filtered (0.1 μm filter, Anotop™, Whatman) 0.1 Mammonia solution.

Hydrothermal Synthesis

The seeded supports were treated in a hydrolyzed synthesis mixture withthe molar composition 3TPAOH:25SiO₂:1500H₂O:100EtOH which was heatedduring 30 h in silicone oil at a temperature of 100° C. After cooling,the membrane was rinsed in 0.1 M ammonia solution. The resultantmembrane was calcined.

The calcined membrane showed the following performance when tested asabove but without hydrogen balance, and averaged over t=1-2 hours and asweep flow of 200 ml/min:

Q₁₀₀=1.1×10⁻⁷ mole/m².s.Pa (102 kg(px)/m².day.bar(px))

α₁₀₀=13.2

The test was then continued at higher pressure, and the system wasadjusted to obtain hydrogen balance which was obtained at 19.5 hours;the performance averaged over t=19.5-20.5 hours was:

Q₅₀₀=2.75×10⁻⁸ mole/m².s.Pa (25 kg(px)/m².day.bar(px))

α₅₀₀=4.8

EXAMPLE 6

A membrane was prepared as in the Example 10, with the exception thatthe crystallization time was 72 hours

Reparation of Membrane

A hydrolyzed synthesis mixture, as used for crystallizing the layer (seeExample 10), was applied to the disk by spincoating at 8000 rpm. Whilestill spinning, the surface was cleaned using 0.1 M ammonia. The treatedmembrane was put on a holder above the liquid level in a 65 ml autoclavecontaining 10 ml water. The closed autoclave was held at 100° C. for 24hours.

The treated membrane was held at 400° C. in air for 6 hours, heat-up andcool-down rate was 2° C./minute

The resulting membrane showed the following performance after 2 hoursexposure to the hydrocarbon stream:

Q₁₀₀=1.06×10⁻⁷ mole/m².s.Pa (97 kg(px)/m².day.bar(px))

α₁₀₀=17.4

This membrane did not show selectivation.

What is claimed is:
 1. A crystalline molecular sieve layer having aselectivity (α_(x)) for para-xylene over meta-xylene of 2 or greater anda permeance (Q_(x)) for para-xylene of 3.27×10⁻⁸ mole(px)/m².s.Pa(px) orgreater measured at a temperature of ≧250° C. and an aromatichydrocarbon partial pressure of ≧10×10³ Pa.
 2. A crystalline molecularsieve layer as claimed in claim 1 wherein the molecular sieve layer iscarried on a porous support.
 3. A crystalline molecular sieve layer asclaimed in claim 1 wherein the crystalline molecular sieve has beengrown from molecular sieve seeds.
 4. A crystalline molecular sieve layeras claimed in claim 1 wherein the permeance for para-xylene is 5.45×10⁻⁸mole(px)/m².s.Pa(px) or greater.
 5. A crystalline molecular sieve layeras claimed in claim 1 wherein the selectivity for para-xylene overmeta-xylene is 2.5 or greater.
 6. A crystalline molecular sieve layer asclaimed in claim 2 which has a (Δα_(px/mx)/Δt) at t=2 hours of greaterthan
 0. 7. A crystalline molecular sieve layer as claimed in claim 1measured at an aromatic hydrocarbon partial pressure of ≧500×10³ Pa. 8.A crystalline molecular sieve layer as claimed in claim 1 which isreparated and which has after reparation a selectivity (α_(x)) forpara-xylene over meta-xylene of 10 or greater and a permeance (Q_(x))for para-xylene of 4.36×10⁻⁹ mole(px)/m².s.Pa(px) or greater measured ata temperature of ≧250° C. and an aromatic hydrocarbon partial pressureof ≧10×10³ Pa.
 9. A crystalline molecular sieve layer as claimed inclaim 7 measured at a temperature of 360° C. or greater.
 10. Acrystalline molecular sieve layer as claimed in claim 1 wherein thecrystalline molecular sieve is an MFI type molecular sieve.
 11. Aprocess for the manufacture of a crystalline molecular sieve layeraccording to claim 1, which process comprises: a) providing a supporthaving deposited thereon seeds of molecular sieve crystals of averageparticle size of 200 nm or less, b) impregnating the support with animpregnating material before or after deposition of the seeds ofmolecular sieve, c) contacting the impregnated support having seedsdeposited thereon with a molecular sieve synthesis mixture, d)subjecting the impregnated support having seeds deposited thereon tohydrothermal treatment, whilst in contact with the molecular sievesynthesis mixture, to form a crystalline molecular sieve layer on thesupport, and (e) removing the impregnating material from the support.12. A process as claimed in claim 11 wherein a pre-impregnation maskinglayer is applied to the support prior to impregnation and issubsequently removed after impregnation.
 13. A process as claimed inclaim 11 wherein the molecular sieve synthesis mixture when formulatedfor the manufacture of a silicon containing molecular sieve, comprises aH₂O:SiO₂ molar ratio within the range of 7 to 100:1.
 14. A process asclaimed in claim 11 wherein the impregnating material is a hydrocarbonresin.
 15. A process as claimed in claim 11 wherein the impregnatingmaterial is a hydrocarbon wax.
 16. A process as claimed in claim 11wherein the impregnating material is an acrylic resin.
 17. A process asclaimed in claim 11 wherein the average particle size of the molecularcrystal seeds is 100 nm or less.
 18. A process as claimed in claim 11wherein the molecular sieve seed is deposited substantially as amonolayer.
 19. A process as claimed in claim 18 wherein the monolayerseed layer is deposited via the use of charge reversal and a cationicpolymer.
 20. A process as claimed in claim 11 wherein the molecularsieve seed is present as a seed layer has a thickness of 3 μm or less.21. A process as claimed in claim 1 wherein the temperature forhydrothermal synthesis is 100° C. or less.
 22. A process as claimed inclaim 11 wherein the crystalline molecular sieve layer is reparated. 23.A process as claimed in claim 12 wherein the pre-impregnation maskingmaterial is polymethylmethacrylate.
 24. A process for enhancing theselectivity of a crystalline molecular sieve layer as claimed in claim 1which process comprises, (i) exposing such a crystalline molecular sievelayer to a hydrocarbon stream comprising at least two components underpressure for a period of time, such that at least one component isseparated from the stream, and (ii) at some point in time during theexposure increasing the partial pressure of at least one component ofthe hydrocarbon stream.
 25. A selectivity enhanced crystalline molecularsieve layer prepared according to claim
 24. 26. A process for theseparation of at least one component from a hydrocarbon stream whichprocess comprises exposing a molecular sieve as claimed in claim 1, to ahydrocarbon stream comprising at least two components such that at leastone component is separated from the stream.
 27. A process for theseparation of para-xylene from a mixture comprising para-xylene and atleast one other isomer of xylene, which process comprises exposing themixture to a crystalline molecular sieve layer, as claimed in claim 1,at a temperature of ≧250° C. and an aromatic hydrocarbon feed partialpressure of ≧10×10³ Pa.